1. Field of the Invention
The present invention relates to the detection of noxious chemical species by means of chemoselective dendrimeric compounds. More particularly, the invention relates to the detection of toxic or explosive chemical vapors, such as chemical agents or nitroaromatic species, by sorbent materials comprising chemoselective dendrimeric molecules.
2. Description of Related Art
Determining and/or monitoring the presence of certain chemical species within an environment, e.g., pollutants, toxic substances and other predetermined compounds, is becoming of increasing importance with respect to such fields as health, environmental protection, resource conservation, and chemical processes. Devices for the molecular recognition of noxious species or other analytes typically include (1) a substrate and (2) a molecular recognition coating upon the substrate. These devices may be used, for example, in chemical vapor sensing or the selective separation of gases by gas chromatography. Small molecular recognition devices are described in Grate et al., Sensors and Actuators B, 3, 85-111 (1991) and Grate et al., Analytical Chemistry, Vol. 65, No. 14, Jul. 15, 1993, both of which are incorporated herein by reference.
Frequently, the substrate is a piezoelectric material or a waveguide, which can detect small changes in mass. One illustrative example of a device relying upon molecular recognition as a surface is known as a surface acoustic wave (SAW) sensor. SAW devices function by generating mechanical surface waves on a thin slab of a piezoelectric material, such as quartz, that oscillates at a characteristic resonant frequency when placed in a feedback circuit with a radio frequency amplifier. The oscillator frequency is measurably altered by small changes in mass and/or elastic modulus at the surface of the SAW device.
SAW devices can be adapted to a variety of gas-phase analytical problems by designing or selecting specific coatings for particular applications. The use of chemoselective polymers for chemical sensor application is well established as a way to increase the sensitivity and selectivity of a chemical sensor with respect to specific classes or types of analytes. Typically, a chemoselective polymer is designed to contain functional groups that can interact preferentially with the target analyte through dipole-dipole, Van der Waal""s, or hydrogen bonding forces. For example, strong hydrogen bond donating characteristics are important for the detection of species that are hydrogen bond acceptors, such as toxic organophosphorus compounds. Increasing the density of hydrogen bond acidic binding sites in the coating of a sensor results in an increase in sensitivity.
Chemoselective films or coatings used with chemical sensors have been described by McGill et al. in Chemtech, Vol. 24, No. 9, 27-37 (1994). The materials used as the chemically active, selectively absorbent layer of a molecular recognition device have often been polymers, as described in Hansani in Polymer Films in Sensor Applications (Technomic, Lancaster, Pa. 1995). For example, Ting et al. investigated polystyrene substituted with hexafluoroisopropanol (HFIP) groups for its compatibility with other polymers in Journal of Polymer Science: Polymer Letters Edition, Vol. 18, 201-209 (1980) Later, Chang et al. and Barlow et al. investigated a similar material for its use as a sorbent for organophosphorus vapors, and examined its behavior on a bulk quartz crystal monitor device in Polymer Engineering and Science, Vol. 27, No. 10, 693-702 and 703-15 (1987). Snow et al. (NRL Letter Report, 6120-884A) and Sprague et al. (Proceedings of the 1987 U.S. Army Chemical Research Development and Engineering Center Scientific Conference on Chemical Defense Research, page 1241) reported making materials containing HFIP that were based on polystyrene and poly(isoprene) polymer backbones, where the HFIP provided strong hydrogen bond acidic properties. These materials were used as coatings on molecular recognition devices, such as SAW sensors, and showed high sensitivity for organophosphorus vapors. However, both the parent polymers and the HFIP-containing materials were glassy or crystalline at room temperature. Because vapor diffusion may be retarded in glassy or crystalline materials, the sensors produced were slow to respond and recover. Further, these are polymeric materials and, like all polymers, they can vary significantly from batch to batch in precise composition, purity and yield. Additional information is reported in Polym. Eng. Sci., 27, 693 and 703-715 (1987).
Daroux et al., U.S. Pat. No. 5,648,186, issued Jul. 15, 1997, discloses a dendrimeric compounds containing electronegative heteroatoms, such as etheric oxygens, which are capable of associating with the cationic species of a salt. The compounds are useful as components of solid electrolytes for use in solid electrolyte cells. Examples of nucleophilic functional groups are those containing Nxe2x80x94H groups, hydroxyl groups, and thiol groups. Examples of nucleofugal groups are compounds containing three or more halogens, tosylates, or other commonly used leaving groups. The core, arm and branch points of the dendrimers are completely aliphatic.
Balogh et al., U.S. Pat. No. 5,938,934 issued Aug. 17, 1999, discloses silicon-containing dendrimer-based networks prepared from radially layered copoly(amidoamine-organosilicon) dendrimers (PAMAMOS), which have a hydrophilic interior and an organosilicon exterior to complex and/or encapsulate metal cations or elemental metals. A hydrophilic PAMAM or PPI dendrimer is formed first, which comprises a polyamide core, and then is reacted with an organosilicon modifier, which forms the outer layer. Some examples of preferred organosilicon modifiers include (3-acryloxypropyl)methyldimethoxysilane, (3-acryloxypropyl)bis(vinyldimethylsiloxy)-methylsilane, iodomethyldimethylvinylsilane, chloromethyldimethylvinylsilane, other (3-acryloxypropyl)-functional silanes, and other haloalkyl-functional silanes.
Klimash et al., U.S. Pat. No. 6,020,456 issued Feb. 1, 2000, discloses the use of polyamidoamine (PAMAM) dendrimers as reagents in optical devices, electrical devices, catalyst systems, sensors and biosensors. The dendrimers are usually synthesized from a reactive initiator core reagent, such as dibenzyl amine, followed by the generation of the degree of additional growth desired to form arms extending radially from the polyamidoamine core. Prior to being combined with the anchoring core, the arms can be individually modified to provide various functional groups, e.g., hydroxyl, mercapto, nitrile, amide, carboxylic, etc. Klimash et al. are particularly interested in dendrimeric compounds having a single disulfide group at the core, which can be reduced to form two sulfhydryl groups, thus splitting the dendrimeric molecule into two parts, each having a single reactive sulfhydryl group to which other molecules can be bound.
The inventors have now discovered a class of dendrimeric molecules that can be used to produce hydrogen bond acidic coatings for chemical sensor applications. Using dendrimers that are highly functionalized results in significant sensitivity improvements. Further, the chemoselective dendrimeric molecules of the present invention exhibit, not only improved sensitivity to organophosphorus species, but also high selectivity and sensitivity toward nitro-substituted chemical vapors, and are thus also useful for detecting the presence of explosives. Conventional explosives, such as trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and octahydro-1,3,5-trinitro-1,3,5,7-tetrazocine(HMX), may be contained in unexploded munitions, e.g., buried below the surface of the ground. Such munitions exude or leak vapors of the explosive. These vapors are typically concentrated in the surrounding soil and then migrate to the surface where they can be detected by the compounds, devices and methods of the invention.
According to a first aspect of the present invention, there is provided a dendrimeric compound having (1) a core portion; (2) at least one arm extending radially from said core portion; (3) at least one branch extending from each said arm; and (4) each said arm having at least one halogen substituted alcohol positioned at the terminus of at least one of said branches.
According to a second aspect of the invention, there is provided a device for selective molecular detection, the device comprising a sensing portion, wherein the sensing portion includes a substrate having coated thereon a layer, the layer comprising the dendrimeric compound of the invention.
According to another aspect of the invention, there is provided a method of detecting a hydrogen bond accepting vapor, such as a nitroaromatic vapor, comprising the steps of:
(a) contacting the molecules of such a vapor with the sensing portion of the device of the invention;
(b) collecting the molecules in the layer of the device, the molecules altering a specific physical property of the layer; and
(c) detecting the amount of change with respect to the physical property from before the contacting step (a) and after the collecting step (b).
According to yet another aspect of the invention, there is provided a solution for preparing a chemical vapor sensor comprising (a) an amount of the dendrimeric compound of the invention effective to enhance the sensitivity of the sensor to hydrogen bond accepting vapors such as chemical agents or nitroaromatic compounds and (b) a solvent for the dendrimeric compound.